Cambrian explosion

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The Cambrian explosion or Cambrian radiation was the seemingly rapid appearance of most major groups of complex animals around 530 million years ago, as evidenced by the fossil record.^[1][2] This was accompanied by a major diversification of other organisms.^[3] Before about 580 million years ago, most organisms were simple, composed of individual cells occasionally organised into colonies. In the following 70 million to 80 million years, the rate of evolution accelerated by an order of magnitude,^[4] and the diversity of life began to resemble today’s.^[5]

The Cambrian explosion theory has generated extensive scientific debate. The seemingly rapid appearance of fossils in the “Primordial Strata” was noted as early as the mid 19th century,^[6] and Charles Darwin saw it as one of the main objections that could be made against his theory of evolution by natural selection.^[7]

The long-running puzzlement about the appearance of the Cambrian fauna, seemingly abruptly and from nowhere, centers on three key points: whether there really was a mass diversification of complex organisms over a relatively short period of time during the early Cambrian; what might have caused such rapid evolution; and what it would imply about the origin and evolution of animals. Interpretation is difficult due to a limited supply of evidence, based mainly on an incomplete fossil record and chemical signatures left in Cambrian rocks.

Contents 1 History and significance 2 Difficulty of dating the Cambrian 3 Types of evidence 3.1 Body fossils 3.2 Trace fossils 3.3 Geochemical observations 3.4 Comparative anatomy 3.5 Molecular phylogenetics 4 Evidence in rocks 4.1 Explanation of a few scientific terms 4.2 Decline of stromatolites over 1 billion years ago 4.3 Increase in size and spininess of acritarchs 4.4 Trace fossils 1 billion years ago? 4.5 Doushantuo Formation 4.6 Ediacaran organisms 4.7 Change in carbon isotope ratios at Ediacaran-Cambrian boundary 4.8 Ediacaran and Early Cambrian diversification of trace fossils 4.9 Small shelly fauna 4.10 Early Cambrian trilobites and echinoderms 4.11 Burgess shale type faunas 4.12 Early Cambrian crustaceans 4.13 Molluscs, annelids or brachiopods? 4.14 Late Cambrian and early Ordovician organisms 5 Data from molecular phylogenetics 6 How real was the explosion? 6.1 How fast did the main metazoan groups evolve? 6.2 Was there a “riot of disparity” in the early Cambrian? 7 Possible causes of the “explosion” 7.1 Changes in the environment 7.1.1 Increase in oxygen levels 7.1.2 Snowball Earths 7.1.3 Carbon isotope fluctuations 7.2 Developmental Explanations 7.2.1 Origin of the bilaterian developmental system 7.2.2 Small increases in genetic complexity can have large effects 7.2.3 Developmental entrenchment 7.3 Ecological Explanations 7.3.1 Arms races between predators and prey 7.3.2 Increase in size and diversity of planktonic animals 7.4 Theoretical explanations 7.4.1 Lots of empty niches 7.5 Discredited hypotheses 8 An Avalon (Ediacaran) explosion? 9 See also 10 Further reading 11 External links 12 References

[edit] History and significance

Geologists as long ago as Buckland (1784–1856) realised that a dramatic step change in the fossil record occurred around the base of what we now call the Cambrian.^[6] Charles Darwin considered this sudden appearance of many animal groups with few or no antecedents to be the greatest single objection to his theory of evolution: indeed, he devoted a substantial chapter of The Origin of Species to this problem.^[7]

American palæontologist Charles Walcott, who extensively studied North American fossil animals, proposed that an interval of time, the “Lipalian”, was not represented in the fossil record or did not preserve fossils, and that the ancestors of the Cambrian animals evolved during this time.^[8]

The intense modern interest in the subject was sparked by the work of Harry B. Whittington and colleagues, who in the 1970s re-analysed many fossils from the Burgess Shale (see below) and concluded that several were complex but very different from any living animals.^[9] Stephen Jay Gould’s popular 1989 account of this work, Wonderful Life,^[10] brought the matter into the public eye and raised questions about what the explosion represented. While differing significantly in details, both Whittington and Gould proposed that all modern animal phyla had appeared rather suddenly. But other analyses, some more recent and some dating back to the 1970s, argue that complex animals similar to modern types evolved well before the start of the Cambrian.^[11][12][13]

[edit] Difficulty of dating the Cambrian

It has been difficult to work out the chronology of the early Cambrian. Absolute radiometric dates for much of the Cambrian, obtained by detailed analysis of radioactive elements contained within rocks, have only rather recently become available, and for only a few regions.^[14]

Relative dating (A was before B) is often good enough for studying processes of evolution, but this has also been difficult, because of the problems involved in matching up rocks of the same age across different continents, particularly around the internationally-defined Precambrian/Cambrian boundary section.^[15] (the most common technique uses widespread but short-lived fossil species to identify rocks of similar ages)

Therefore dates or descriptions of sequences of events should be regarded with caution until better data become available.

[edit] Types of evidence

[edit] Body fossils

Body fossils preserve significant parts of organisms and are therefore the most informative type of evidence. Unfortunately they are increasingly rare as one looks further back in time, among other reasons because the rocks in which they are buried are usually covered by more recent rocks and because they may have been eroded before being covered by later rocks. One recent study concluded that “parts of the fossil record are clearly incomplete, but they can be regarded as adequate to illustrate the broad patterns of the history of life.”^[16] But there is evidence that some types of animals or parts of animals are relatively likely to be preserved as fossils in some environments and times, and extremely unlikely to be preserved in other environments and times. Part of this is due to changes in the chemistry of the oceans, which were partly caused by the on-going evolution of life, and these changes were most significant before the start of the Cambrian.^[17]

Another limitation in the discovery and use of body fossils is the lack of preservation of large portions of the body. In most cases the sole anatomical features that are fossilized are the highly mineralised body parts containing high proportions of silica (sponges' skeletons), calcium carbonate (the shells of bivalves, gastropods and ammonites and exoskeletons of most trilobites and some crustaceans) or calcium phosphate (the bones of vertebrates). The majority of animal species living now are unlikely ever to leave fossils, since they are soft-bodied invertebrates such as worms and slugs. Of the more than 30 phyla of living animals, two-thirds have never been found as fossils.^[18]

The Cambrian fossil record includes an unusually high number of lagerstätten which preserved the fossils' soft tissues in extremely fine detail. This has allowed paleontologists to examine the internal workings of animals which in other sediments are only represented by shells, spines, claws, etc. The most significant Cambrian lagerstätten are: the early Cambrian Maotianshan shale beds of Chengjiang (Yunnan, China) and Sirius Passet (Greenland)^[19]; the middle Cambrian Burgess Shale (British Columbia, Canada)^[20]; and the Upper Cambrian Orsten (Sweden) fossil beds.

While lagerstätten are superior to most fossil beds in preserving fine anatomical detail, they are far from perfect. The majority of then-living animals are probably not represented because lagerstätten are restricted to a narrow range of environments (e.g. where soft-bodied organisms can be preserved very quickly by processes such as mudslides), and the exceptional events that cause quick burial make it difficult to study the normal environments of the animals.^[21] In addition, the known lagerstätten cover only a very limited period of time within the Cambrian, and none covers the crucial period just before the start of the Cambrian. Because normal fossil beds are very rare and lagerstätten even rarer, both are very unlikely to show the first occurrence of any type of organism.^[22]

[edit] Trace fossils

Trace fossils consist mainly of tracks and burrows on and under what was then the seabed.

Trace fossils are particularly significant because they represent a data source that is not limited to animals with easily-fossilized hard parts, and which reflects organisms' behaviour. Also many traces date from significantly earlier than the body fossils of animals that are thought to have been capable of making them.^[23] Whilst exact assignment of trace fossils to their makers is generally impossible, traces may provide the earliest physical evidence of the appearance of moderately complex animals (comparable to earthworms).

[edit] Geochemical observations

Main article: Early Cambrian geochemical fluctuations

The chemistry of rocks deposited around the Cambrian boundary reflects the environmental conditions of the time.

Several chemical markers speak of a drastic change in the environmental parameters, which are consistent with a mass extinction, or a massive warming resulting from the release of methane ice.

Such changes may have been a cause, or effect, of the Cambrian explosion. They help to constrain hypotheses by giving scientists a rough approximation of what the globe was doing at this time.

[edit] Comparative anatomy

Cladistics is a technique for working out the “family tree” of a set of organisms, and has most often applied to evidence from comparative anatomy (features of the bodies of organisms). In this kind of analysis it is possible to include both living and fossilized organisms and work out their evolutionary relationships. Sometimes one can conclude that group A must have evolved before groups B and C, because B and C have more similarities to each other than either has to A. On its own this method can say nothing about when A evolved, but if there are fossils of B or C dating from X million years ago, then A must have evolved more than X million years ago.

[edit] Molecular phylogenetics

Molecular phylogenetics attempts to reconstruct the relationships between organisms by comparing details of their biochemistry, such as their DNA. In other words, it applies the analysis techniques of cladistics to biochemical rather than anatomical features. It provides an alternative line of evidence about evolution in the Cambrian and Precambrian, although the need for calibration against the fossil record means it is not entirely independent. Further, since the “clocks” measure molecular evolution, a period of rapid evolution is indistinguishable from a longer period of slow change, so it is unwise to rely on molecular phylogeny for estimates of dates^[24].

[edit] Evidence in rocks

This lists the main items in order of the time when the relevant rocks were formed, because timing is the central issue in the Cambrian explosion – but remember that dating rocks from the Cambrian and earlier rocks is very difficult. The survey also starts well before the start of the Cambrian and finishes in the early Ordovician, because some scientists think that the diversification of animal life started before and finished after the Cambrian.^[25]

It covers body fossils, trace fossils and geochemical evidence, because these are all found in rocks which can be dated at least approximately. Arguments based on molecular phylogenetics will appear in a separate section, because this type of evidence is much harder to date with confidence.

[edit] Explanation of a few scientific terms

To avoid becoming even longer this article uses some scientific terms, and this is a good place for some simple explanations.^[26]

Phylum is the highest level in the Linnean system for classifying animals. Phyla can be thought of as groupings of animals based on general body plan.^[27] Despite the seemingly different external appearances of organisms, they are classified into phyla based on their internal organizations.^[28] For example despite their obvious differences spiders and crabs both belong to the phylum Arthropoda; but earthworms and tapeworms, although similar in shape, are members of the Annelida and Platyhelminthes respectively.

But the word "phylum" does not describe a fundamental division of nature (not like the difference between electrons and protons). It simply refers to a very high level in the classification system created by Linnaeus in the 18th century to describe all the animals which are alive to-day. This system is not perfect even for modern animals: different books quote different numbers of phyla, mainly because they disagree about the classification of a huge number of worm-like species. Classification systems based on living organisms, including Linneus', do not accommodate extinct organisms well, or even at all.^[18][29]

Triploblastic means consisting of 3 layers, which are formed in the embryo (quite early in the animal's development from a single-celled egg to a larva or juvenile form). The innermost layer forms the digestive tract (gut); the outermost forms skin; and the middle one forms muscles and all the internal organs except the digestive system. Most types of living animal are triploblastic – the best-known exceptions are Porifera (sponges) and Cnidaria (jellyfish, sea anemones, etc.).

Bilaterian means having 2 sides; this implies that they also have top and bottom surfaces and, perhaps more importantly, distinct front and back ends. All known bilaterian animals are triploblastic, and all known triploblastic animals are bilaterian except for echinoderms (but sea cucumbers do have distinct front and back ends; and echinoderm larvae have 2 sides). Porifera (sponges) and Cnidaria (jellyfish, sea anemones, etc.) are radially symmetrical (like wheels).

Coelomate means having a body cavity (coelom) which contains the internal organs. Most of the phyla featured in the debate about the Cambrian explosion are coelomates: arthropods, annelid worms, molluscs, echinoderms and chordates (which includes us vertebrates) – the non-coelomate priapulids are an important exception. All coelomate animals are triploblastic, but some triploblastic animals do not have a coelom (e.g. flatworms; their organs are surrounded by unspecialized tissues). Some bilaterian animals are not coelomates (e.g. flatworms). Echinoderms are coelomates; living species do not look bilaterian (they are radially symmetrical, although sea cucumbers have distinct front and rear ends), but the earliest echinoderms are still poorly understood and some may have been bilaterally symmetrical.^[30]

[edit] Decline of stromatolites over 1 billion years ago

Stromatolites are stubby pillars of sediment built by photosynthesizing microorganisms, especially cyanobacteria. They are now restricted to hostile environments such as extremely salty lagoons: in more amenable environments, they are eliminated by grazing and burrowing invertebrates.

Stromatolites are a major constituent of the fossil record for about the first 3 billion years of life on earth, with their abundance^{[verification needed]} peaking about 1250 million years ago. They subsequently declined in abundance and diversity, which by the start of the Cambrian had fallen to 20% of their peak. The most widely-supported explanation is that stromatolite builders fell victims to grazing creatures: implying that sufficiently complex organisms were common over 1 billion years ago.^[11][12][31]

The connection between grazer and stromatolite abundance is well documented in the younger Ordovician evolutionary radiation; stromatolite abundance also increased after the end-Ordovician and end-Permian extinctions decimated marine animals, falling back to earlier levels as marine animals recovered.^[32]

[edit] Increase in size and spininess of acritarchs

Acritarchs include the remains of a wide range of quite different kinds of organisms – ranging from the egg cases of small metazoans to resting cysts of many different kinds of chlorophyta (green algae). They first appear in rocks about 2 billion years old, but about 1 billion years they started to increase in abundance, diversity, size, complexity of shape and especially size and number of spines. Their populations crashed during the Snowball Earth episodes, when all or very nearly all of the Earth's surface was covered by ice or snow, but they reached their highest diversity in the Paleozoic era (i.e. after the start of the Cambrian). Their increasingly spiny forms in the last 1 billion years possibly resulted from the need for defense against predators, especially predators large enough to swallow them or tear them apart. Other groups of small organisms from the Neoproterozoic era also show signs of anti-predator defenses.^[31]

Further evidence that predation, or at least herbivory, on plankton first appeared around this time comes from a consideration of taxon longevity. The abundance of planktonic organisms that evolved between 1,700 and 1,400 million years ago were limited by nutrient availability – a situation which limits the origination of new species because the existing organisms are so specialised to their niches, and no other niches are available for occupation. Around about 1,000 million years ago, species longevity fell sharply, suggesting that predation pressure, probably by protist herbivores, became an important factor. Predation would have kept populations in check, meaning that some nutrients were left unused, and new niches were available for new species to occupy.^[33]

[edit] Trace fossils 1 billion years ago?

Marks found in rocks about 1 billion years old in India may have been made by creatures moving across and below soft surfaces. The organisms making the traces were clearly not exploiting deep sediments, but only the layers immediately below the mat of cyanobacteria that covered the seabed. The researchers concluded that the burrows were produced by the peristaltic action of triploblastic metazoans up to 5 mm wide — in other words by animals that: resembled earthworms in size, method of burrowing and internal complexity; and may have been coelomates.^[34] But other researchers have dismissed this and other purported finds of trace fossils older than about 600 million years ago, usually on the grounds that they were produced by physical processes rather than by organisms.^[35]

[edit] Doushantuo Formation

The Doushantuo Formation in China contains one of the oldest known lagerstätten. These rocks range from about 635 million to about 551 million years ago, but their animal fossils are mostly less than 580 million years old, predating by perhaps 5 million years the earliest of the 'classical' Ediacaran faunas (see below) from Mistaken Point, Newfoundland.^[36] Doushantuo fossils are all marine, microscopic and highly preserved. They include algae, giant acritarchs and what may be phosphatised embryos of bilaterian animals; but some scientists think the “embryos” are fossils of giant sulfur-metabolising bacteria like Thiomargarita, which is so large that it is visible to the naked eye.^[37]

One Doushantuo fossil from about 580M years ago, Vernanimalcula (0.1 to 0.2 mm in diameter), has been described as a possible adult triploblastic coelomate bilaterian, in other words about as complex as an earthworm or a mollusc;^[38] others think it was more probably created by non-biological rock-forming processes;^[39] but the team that discovered Vernanimalcula have defended their conclusion that it was an animal, pointing out that they found 10 specimens of the same size and configuration, and stating that non-biological processes would be very unlikely to produce so many specimens that were so alike.^[40]

The most recent Doushantuo rocks show a sharp decrease in the ¹³C/¹²C carbon istope ratio. Since this change appears to be worldwide but its timing does not match that of any other known major event such as a mass extinction, it may represent “possible feedback relationships between evolutionary innovation and seawater chemistry” in which metazoans (multi-celled organisms) removed carbon from the water, this increased the concentration of oxygen, and the increased oxygen level made possible the evolution of new metazoans.^[36]

[edit] Ediacaran organisms

Main articles: Ediacara biota, Cloudinid, Kimberella, and Spriggina

Strange-looking fossils are known from the Ediacaran period, dating from 610 million years ago to the start of the Cambrian. Most of these Ediacaran biota were at least a few centimeters long, significantly larger than any earlier fossils. Three distinct assemblages (sets) of Ediacaran fossils are recognised, increasing in size and complexity as time progresses. The most diverse included a range of discs, fronds and seemingly segmented forms.^[41]

Many of these organisms were quite unlike anything that appeared before or since, resembling discs, mud-filled bags, or quilted mattresses – one palæontologist proposed that the strangest organisms should be classified as a separate kingdom, Vendozoa.^[42]

At least some may have been early forms of the phyla at the heart of the "Cambrian explosion" debate, having been interpreted as early molluscs (Kimberella),^[43][13] echinoderms (Arkarua);^[44] and arthropods (Spriggina,^[45] Parvancorina).^[46] However there is still debate about the classification of these specimens, mainly because the diagnostic features which allow taxonomists to classify more recent organisms are generally absent in the Ediacarans.^[47]

By the end of the Ediacaran period, a small fauna of calcifying organisms had evolved. These organisms, such as Cloudina and Namapokia, are as difficult to classify as any other Ediacaran, but bear some resemblance to polychaete worms.^[48][49][50] These Precambrian organisms show evidence of two postulated causes of the explosion: calcareous shells, and predation. Cloudina survived selective predation: Predatory organisms bored into Cloudina, but not the neighbouring Sinotubulites.^[51]

[edit] Change in carbon isotope ratios at Ediacaran-Cambrian boundary

Carbon has 2 stable isotopes, carbon-12 (¹²C) and carbon-13 (¹³C). At the boundary between the Ediacaran and Cambrian periods the ratio of ¹³C to ¹²C drops sharply, and then is unusually erratic until the mid-Cambrian. There is no easy explanation for the rapid variation of the ratio in the first half of the Cambrian, and at present it is impossible to decide between the two widely-supported explanations for the sharp drop at the Ediacaran-Cambrian boundary, a mass extinction or a methane “burp”.^[52]

[edit] Ediacaran and Early Cambrian diversification of trace fossils

The earliest Ediacaran fossils (Assemblage 1 above), 610–600M years ago, contain only cnidarian resting traces. Around 565M years ago (Ediacaran Assemblage 2 above) more complex trace fossils appear, which require a body plan with a hydrostatic skeleton against which muscles pull, i.e. more complex body structures than those of cnidarians or flatworms.^[41]

Around the start of the Cambrian (about 543 million years ago) many new types of traces first appear, including well-known vertical burrows such as Diplocraterion and Skolithos, and traces normally attributed to arthropods, such as Cruziana and Rusophycus. The vertical burrows indicate that worm-like animals acquired new behaviors and possibly new physical capabilities. If traces such as Cruziana and Rusophycus were produced by arthropods, that would indicate that arthropods or their immediate predecessors had developed exoskeletons, although not necessarily as hard as they became later in the Cambrian.^[35]

[edit] Small shelly fauna

Fossils known as “small shelly fauna” have been found in many parts on the world, and date from just before the Cambrian to about 10 million years after the start of the Cambrian (the Nemakit-Daldynian and Tommotian ages; see timeline). These are a very mixed collection of fossils: spines, sclerites (armor plates), tubes, archeocyathids (sponge-like animals) and small shells very like those of brachiopods and snail-like molluscs – but all tiny, mostly 1 to 2 mm long.^[53]

[edit] Early Cambrian trilobites and echinoderms

The earliest Cambrian trilobite fossils are about 530 million years old, but even then they were quite diverse and world-wide, which suggests that these arthropods had been around for quite some time.^[54]

The earliest generally-accepted echinoderms appeared at about the same time, although it has been suggested that some fossils from the Ediacaran period were echinoderms (see above). The early Cambrian Helicoplacus was a cigar-shaped creature up to 7 cm long that stood upright on one end. Unlike modern echinoderms it was not radially symmetrical with the mouth at the center, but had a spiral food groove on the outside along which food was moved to a mouth that is thought to be located on the side.^[55]

[edit] Burgess shale type faunas

Main article: Burgess shale type preservation

The Burgess Shale is one of palaeontology's most famous fossil assemblages. Usually, only the hard parts of organisms are preserved – their bones, teeth and shells, for instance. However, in truly exceptional circumstances, organisms can be buried so quickly in oxygen starved sediments that the usual processes of rotting and decay do not break down their soft parts. In the presence of the right bacteria, this soft tissue can be mineralised and preserved, perhaps in the form of a pyrite film or a phosphatic "blob".

Exactly such conditions were present around 505 million years ago in the deep seas which would later become British Columbia, Canada. A storm or strong current swept organisms from the shallow sea floor in a cloud of mud. As it settled, the fauna was preserved in exquisite detail.

This assemblage, along with others across the globe, gives palaeontologists a unique glimpse into the diversity present immediately after the dust of the Cambrian explosion had settled. A vast array of initially baffling organisms are preserved – the bizarre appearance of one inspired its name, Hallucigenia.

Early workers in the field tried to shoehorn the organisms into extant phyla; the shortcomings of this approach led them to erect a multitude of new phyla to accommodate all the oddballs. It has since been realised that these lineages represent early offshoots from the phyla we know today – slightly different designs fated to perish rather than flourish into phyla, as their cousin lineages did.

Estimates of the diversity of this early fauna thus place it as broadly comparable to today's, and its disparity - the amount of distinct "body plans" - is also surprisingly close to that of more recent faunas.

[edit] Early Cambrian crustaceans

Crustaceans, one of the three great modern groups of arthropods, are very rare throughout the Cambrian. Convincing crustaceans were once thought to be common in Burgess shale-type biotas, but none of these individuals can be shown to fall into the crown group of "true crustaceans".^[56] The Cambrian record of Crustaceans comes from microfossils: the miniature organisms of the Orsten, and similar phosphatised horizons, provide a window on later Cambrian crustacea; however, the taphonomic mode only preserved organisms smaller than 2mm, limiting the data sert to juviniles and miniaturised adults.

A more informative data source is the organic microfossils of the Mount Cap formation, Canada. This late Early Cambrian assemblage (510 to 515 million years ago) consists of microscopic fragments of arthropods' cuticle, which is left behind when the rock is dissolved with a strong acid. The diversity of this assemblage is similar to that of modern crustacean faunas. Most interestingly, analysis of fragments of feeding machinery found in the formation shows that it was adapted to feed in a very precise and refined fashion. This contrasts with most other early Cambrian arthropods, which fed messily by shovelling anything they could get their feeding appendages on into their mouths. This sophisticated and specialised feeding machinery belonged to a large (~30 cm)^[57] organism, and would have provided great potential for diversification: specialised feeding apparatus allows a number of different approaches to feeding to develop, and creates a number of different approaches to avoiding being eaten!^[56]

[edit] Molluscs, annelids or brachiopods?

Wiwaxia, found so far only in the Burgess Shale, had chitinous armor consisting of long vertical spines and short overlapping horizontal spines. It also had what looked like a radula (chitinous toothed “tongue”), a feature which is otherwise only known in molluscs. Some researchers think the pattern of its scales links its closely to the annelids (worms) or more specifically to the polychaetes (“many bristles”; marine annelids with leg-like appendages); but others disagree.^[58][59]

Orthrozanclus, also discovered in the Burgess Shale, had long spines like those of the wiwaxiids, and small armor plates plus a cap of shell at the front end like those of the halkieriids. The scientists who described it say it may have been closely related to the halkieriids and the wiwaxiids.^[60]

Halkieria resembled a rather long slug, but had a small cap of shell at each end and overlapping armor plates covering the rest of its upper surface – the shell caps and armor plates were made of calcium carbonate. Its fossils are found on almost every continent in early to mid Cambrian deposits, and the “small shelly fauna” deposits contain many fragments which are now recognized as parts of Halkieria’s armor. Some researchers have suggested that halkieriids were closely related to the ancestors of brachiopods (the structure of halkieriids' front and rear shell caps resembles that of brachiopod shells) and to the wiwaxiids (the pattern of the scale armor over most of their bodies is very similar).^[61] Others think the halkieriids are closely related to molluscs and have a particularly strong resemblance to chitons.^[62]

Odontogriphus is known from almost 200 specimens in the Burgess Shale. It was a flattened bilaterian up to 12 cm (5 in) long, oval in shape, with a ventral U-shaped mouth surrounded by small protrusions. The most recently found specimens are very well preserved and show what may be a radula, which led those who described these specimens to propose that it was a mollusc.^[63] But others disputed the finding of a radula and suggested Odontogriphus was a jawed segmented worm belonging to the Lophotrochozoa (a “super-phylum” which contains the annelids, brachiopods, molluscs and all other descendants of their last common ancestor).^[47]

[edit] Late Cambrian and early Ordovician organisms

Right up to the end of the Cambrian there were high levels of "disparity" (sets of organisms with significantly different “designs”) but low levels of diversity (total numbers of species or genera). Indeed, Cambrian and Ordovician arthropod communities were no less disparate than today's.^[64]

There was a mass extinction at the Cambrian-Ordovician boundary, and typical Paleozoic marine diversity and ecosystems appear during the recovery from the extinction.^[26] It is also worth noting that the earliest fossils of one phylum, the Bryozoa, first appear in the Ordovician period.

[edit] Data from molecular phylogenetics

A study in 1996 concluded that the genetic "family tree" of organisms indicates that protostomes (including the ancestors of molluscs, annelids and arthropods) diverged from deuterostomes (which includes the ancestors of chordates and echinoderms) about a billion years ago, almost twice as long ago as the start of the Cambrian; that, within the deuterostome group, chordates diverged from echinoderms some time later; and that the evolution of animal phyla was a long process.^[65] A later study in 1998 found flaws in the first one and concluded that protostomes diverged from deuterostomes about 670M years ago and that chordates diverged from echinoderms about 600M years ago.^[66]

There is still debate about the interpretation of data from molecular phylogenetics. For example: one analysis in 2003 concluded that protostomes and deuterostomes diverged 582 ± 112 M years ago (note the wide margin of uncertainty; for example 582-112 = 470M years ago, after the end of the Cambrian);^[67] another in April 2004 concluded that the last common ancestor of bilaterians arose between 573M and 656M years ago, i.e. around the start of the Ediacaran period; ^[68] and a third in November 2004 concluded that the 2 previous ones were faulty and that protostomes and deuterostomes diverged 786M to 1,166M years ago, i.e. well before the start of the Ediacaran period.^[69]

[edit] How real was the explosion?

[edit] How fast did the main metazoan groups evolve?

In Darwin’s time what was known of the fossil record seemed to suggest that the major metazoan groups appeared in a few million years of the early to mid-Cambrian, and even in the 1980s this still appeared to be the case.^[9][10] But more recently-discovered fossil evidence suggests that at least some triploblastic bilaterians were present before the start of the Cambrian: Kimberella left the kind of fossils one would expect of an early mollusc, and the scratches on the rocks near these fossils suggest a mollusc-like method of feeding (555M years ago);^[13] and if Vernanimalcula was a triploblastic bilaterian coelomate, it would prove that moderately complex animals appeared even earlier (600-580M years ago).^[38][39][40] The presence of borings in shells of Cloudina suggests there were sufficiently advanced predators in the late Ediacaran period.^[51] Some mid-Ediacaran trace fossils appear to have been produced by animals more complex than flatworms and having hydrostatic skeletons, about 565M years ago.^[41]

Further back in time, the long decline of stromatolites after about 1250 million years ago suggests that animals sufficiently complex to graze on bacterial mats were abundant well before the Ediacaran period;^[11] and the increase in abundance, diversity and spininess of acritarchs in the same period suggests that there were sufficient predators large enough to make such defenses necessary.^[31]

At the other end of the critical time range, several major modern types of animal did not appear until the late Cambrian, while typical Paleozoic ecosystems did not appear until the Ordovician.^[26]

So the evidence no longer appears to support the view that animals of "modern" complexity (comparable to living invertebrates) appeared in a few million years of the early to mid-Cambrian. But most modern phyla first appear in the Cambrian (except for possible molluscs, echinoderms and arthropods in the Ediacaran), and the rise in disparity (wide range of animals with significantly different "designs") seems to have occurred mostly in the early Cambrian.^[26]

[edit] Was there a “riot of disparity” in the early Cambrian?

In this context “disparity” means a wide range of animals with significantly different “designs”; while “diversity” means total number of genera or species and says nothing about the number of different basic “designs” (there could be many variations on the same few designs). There is little doubt that disparity rose sharply in the early Cambrian and was exceptionally high for the rest of the Cambrian – we see modern-looking animals such as crustaceans, echinoderms, and fish at about the same time and often in the same fossil beds as creatures like Anomalocaris and Halkieria, which are currently regarded as “aunts” or “great-aunts” of modern groups.^[26]

On closer examination we find another surprise – some modern-looking animals, e.g. the early Cambrian crustaceans, trilobites and echinoderms, appear earlier in the fossil record than some of the “aunts” or “great-aunts” of modern groups.^{[70][71][54][55]} This could be a result of gaps in the fossil record or of preservational biases in different environments; or it could mean that the ancestors of various modern groups evolved at different times and possibly at different speeds.^[26]

[edit] Possible causes of the “explosion”

Despite the evidence that moderately complex animals (triploblastic bilaterians) existed before and possibly long before the start of the Cambrian, it seems that the pace of evolution was exceptionally fast in the early Cambrian. Naturally there has been a lot of discussion about why this should have happened.

[edit] Changes in the environment

[edit] Increase in oxygen levels

Earth’s earliest atmosphere contained no free oxygen; the oxygen that animals breathe today, both in the air and dissolved in water, is the product of billions of years of photosynthesis, mainly by microorganisms such as cyanobacteria. The concentration of oxygen in the atmosphere has risen gradually (with a few ups and downs) over about the last 2.5 billion years (before that oxygen-hungry elements such as iron reacted with all the oxygen that was produced).^[18]

Shortage of oxygen might well have prevented the rise of large, complex animals for a long time. The amount of oxygen an animal can absorb is largely determined by the area of its oxygen-absorbing surfaces (lungs and gills in the most complex animals; the skin in less complex ones); but the amount needed is determined by its volume, which grows faster than the oxygen-absorbing area if an animal’s size increases equally in all directions. An increase in the concentration of oxygen in air or water would reduce or remove this difficulty. But apparently there was already enough oxygen to support reasonably large “Vendobionta” in the Ediacaran period.^[72] Perhaps a further increase in oxygen concentration was required to give animals the energy to produce substances such as collagen which are needed for the construction of complex structures, particularly those used in predation and defense against predation.^[73]

[edit] Snowball Earths

There is plenty of evidence that in the late Neoproterozoic (extending into the early Ediacaran period) the Earth suffered massive glaciations in which most of its surface was covered by ice and temperatures were around freezing even at the Equator. Some researchers argue that these may have been an important factor in the Cambrian explosion, since the earliest known fossils of animals appear shortly after the last "Snowball Earth" episode.^[74]

But it is hard to see how such catastrophes could have led to increases in the size and complexity of animals without clear evidence of a causal mechanism.^[26] Perhaps the cold temperatures increased the concentration of oxygen in the oceans—the solubility of oxygen nearly doubles as seawater cools from 30 °C to 0 °C.^[75] On the other hand they may have delayed the evolution of existing metazoans to larger sizes.^[31]

[edit] Carbon isotope fluctuations

As we've already seen, there was a very sharp decrease in the ¹³C/¹²C ratio at the Ediacaran-Cambrian boundary, followed by unusually strong fluctuations throughout the early Cambrian. Many scientists assume that the initial sharp drop represents a mass extinction at the start of the Cambrian.^[72][76] It might even have caused a mass extinction – the Permian–Triassic extinction event is associated with a similar sharp decrease in the ¹³C/¹²C ratio; this is usually explained as due to massive dissociation of methane clathrates, and it is widely thought that the resulting methane emissions triggered severe global warming and other environmental catastrophes. And the ¹³C/¹²C fluctuations in the early Cambrian resemble those of the early Triassic, when life was struggling to recover from the Permian-Triassic extinction.^[77]

But it’s difficult to see how a mass extinction could have triggered a sharp increase in disparity and diversity. Mass extinctions such as the Permian-Triassic and Cretaceous–Tertiary raised existing animals from insignificance to “dominance”, but these replaced different but similarly complex animals that were dominant before these extinctions, and there was no increase in disparity or diversity.^[26]

Others have suggested that each short-term decrease in the ¹³C/¹²C ratio through out the early Cambrian represents a methane “burp” which, by raising global temperatures, triggered an increase in diversity.^[78] But this hypothesis also fails explain the increase in disparity.^[26]

[edit] Developmental Explanations

Some theories are based on the idea that relatively small changes in the way in which animals develop from embryo to adult may have produced very rapid evolution of body forms. Unfortunately such theories do not explain why the origin of such a development system should by itself lead to increased diversity or disparity. In fact if at least one Ediacaran is a bilaterian (for example Kimberella, Spriggina or Arkarua), then the bilaterian developmental system existed at least a few tens of millions of years before the Cambrian "explosion", which suggests that something else might be needed to account for the "explosion".^[26]

[edit] Origin of the bilaterian developmental system

Hox genes regulate the operation of other genes by switching them on or off in various parts of the body, for example “make an eye here” or “make a leg there”. Very similar Hox genes are found in all animals from Cnidaria (e.g. jellyfish) to humans, although mammals have 4 sets of Hox genes while Cnidaria have only one.^[79] Hox genes in different animal groups are so similar that, for example, one can transplant a human “make an eye” Hox gene into a fruitfly embryo and it still causes an eye to form – but it’s a fruitfly eye, because the other genes that the transplanted Hox gene activates are fruitfly genes.^[80]

The fact that all animals have such similar Hox genes strongly suggests that the last common ancestor of all bilaterians had similar Hox genes. But this does not mean that the last common ancestor of bilaterians had anatomical features that resembled those of any living animal, since for example the same Hox gene can produce structures as different as a human eye and an insect eye. It’s more likely that the various bilaterian lineages became separate before they were committed to any specific way of building specific organs, and therefore that their last common ancestor was small, very simple, and probably rather delicate. This suggests that it will be very difficult to find fossils of the last common ancestor of all bilaterians.^[79]

[edit] Small increases in genetic complexity can have large effects

In most organisms that reproduce sexually, each child gets 50% of its genes from each parent. This means that a small increase in the complexity of the genome can produce a wide increase in the range of variations in body form.^[81] (rather like the way you can deal a larger number of unique hands if you increase the number of different cards in the deck). Much of biological complexity probably arises from the operation of relatively simple rules within large numbers of cells functioning as cellular automata.^[82] (a simple example would be Conway's Game of Life, where complex and often surprising patterns are produced by cells that follow very simple rules)

[edit] Developmental entrenchment

Several scientists suggest that, as organisms become more complex, the developmental stages that produce the body plans are overlain with "down-stream" genetic mechanisms that produce more specific body components, and that this makes it progressively less likely that modifications of the "up-stream" stages will pass the tests of natural selection. So the developmental stages when the phylum-level body plans are laid down become entrenched and the body plans become frozen in place.^[83] (rather like the fact that it is much easier to change a building's fixtures and fittings than to change its structure, especially its foundations) Conversely, major modifications are "easier" in the early stages of the evolution of a major clade. But the author of this idea has more recently argued that this "entrenchment" is not a major factor.^[84]

The fossil evidence relating to this idea is also ambiguous. It has long been noted that variation within a species is often largest in the earliest members of a clade. For example some Cambrian trilobite species have varying numbers of thoracic segments, but later trilobite species show much less variation in this respect.^[26] But a Silurian trilobite species has been found which has as much variation in number of thoracic segments as the Cambrian species. Researchers have suggested that the general decrease in variability was caused by ecological or functional constraints; for example, one might expect a less variable number of segments once trilobites developed rolling up like modern pillbugs as a form of defense.^[85]

[edit] Ecological Explanations

These focus on the interactions between different types of organism. Some of these hypotheses deal with changes in the food chain; some suggest arms races between predators and prey, which might have driven the evolution of hard body parts in the early Cambrian; and some focus on the more general mechanisms of coevolution (a simple more recent example is the ways in which flowering plants and the insects which pollinate them have adapted to each other). Such theories are well suited to explaining why there was a rapid increase in both disparity and diversity, and the challenge for them is to explain why the "explosion" happened at that particular time.^[26]

[edit] Arms races between predators and prey

Predation by definition means that the prey dies, so one would expect that it would be one of the strongest components of natural selection. The pressure to adapt should be stronger on the prey than on the predator, because the predator lives to hunt again if it "loses a contest" (this is known as the "life-dinner" principle - the predator only risks losing one meal).^[86]

But there is enough evidence of predation well before the start of the Cambrian, for example the increasingly spiny forms of acritarchs and the holes drilled in Cloudina shells. Hence it is unlikely that predation triggered the Cambrian "explosion", although it very likely had a strong influence on the body forms that the "explosion" produced.^[31] (but see below for a more complex set of processes that may have been triggered by predation)

[edit] Increase in size and diversity of planktonic animals

Geochemical evidence strongly indicates that the total mass of plankton has been similar to modern levels since early in the Proterozoic. But before the start of the Cambrian the plankton made no contribution to the food supply of organisms at greater depths, because their corpses and droppings were too small to fall quickly towards the sea-bed (their "drag" was about the same as their weight) and so they were eaten by other plankton or destroyed by chemical processes before they could become food for necktonic and benthic animals (swimmers and sea-bottom crawlers).

Early Cambrian fossils have been found of mesozooplankton (mid-sized planktonic animals, barely large enough to see without magnification) that were well-equipped for filter-feeding on microscopic plankton (mostly phytoplankton, i.e. planktonic "plants"). The new mesozooplankton would have produced droppings and corpses that were large enough to fall fairly quickly; if they were eaten, they provided food for necktonic and benthic animals, which could therefore become larger and more diverse; if the falling particles reached the sea-floor without being eaten, they would be buried and this would increase the concentration of oxygen in the water by reducing the concentration of carbon (carbon is an "oxygen-hungry" element) - in other words, the appearance of mesozooplankton increased the supply of both food and oxygen, and thus made it possible for larger, more diverse necktonic and benthic animals to evolve. The rise of herbivorous mesozooplankton would also have created an ecological niche for even larger carnivorous mesozooplankton, whose corpses and droppings would have produced a further increase in the food and oxygen available.^[3]

The initial herbivorous mesozooplankton were probably larvae of benthic animals, and the evolution of planktonic larvae of benthic animals was probably a consequence of the increasing level of predation at the sea-floor in the Ediacaran period.^[3][77]

[edit] Theoretical explanations

Several scientists have produced theoretical models of what might have caused the Cambrian explosion. Of course these models cannot prove what did happen, but a model whose "predictions" match the known fossil evidence may help paleontologists by prompting them to look for evidence that matches the model's assumptions (such evidence may be new, or may be new interpretations of known fossils).

[edit] Lots of empty niches

Valentine has argued in several papers that it's reasonable to assume that: significant changes in body form are "difficult"; a new major innovation has much more chance of being successful if it faces little or no competition for the ecological niche that it is trying to occupy, so that the prospective new type of organism has enough time to adapt well to its new niche (a simple modern analogy would be that golfers who change their swings have a short-term loss of form before they start getting the benefits). This would imply that major innovations are much more likely to succeed during the early stages of the diversification of animals, because that diversification fills almost all the ecological niches.^[84] It also implies that there is a wide range of other potential phyla, but the lack of empty niches prevents them from developing. Valentine's model does make it easy to understand why the Cambrian explosion happened only once and why its duration was limited.^[26]

[edit] Discredited hypotheses

Main article: Discredited hypotheses for the Cambrian explosion

As our understanding of the events of the Cambrian becomes clearer, data has accumulated to make some hypotheses look improbable. Causes that have been proposed but are now discounted include the evolution of herbivory, vast changes in the speed of tectonic plate movement or of the cyclic changes in the Earth's orbital motion, or the operation of different evolutionary mechanisms from those that are seen in the rest of the Phanerozoic eon.

[edit] An Avalon (Ediacaran) explosion?

In early 2008 a team analysed the "disparity" of Ediacaran organisms from 3 different fossil beds: Avalon in Canada (575 to 565M years ago), White Sea in Russia (560 to 550M years ago) and Nama in Namibia (550 to 542M years ago, immediately before the start of the Cambrian). They found that, while the White Sea assemblage had the most species, there was no significant difference in disparity between the 3 groups, and concluded that these organisms must have gone through their own "explosion" at the beginning of the Avalon timespan.^[87]

[edit] See also

• Burgess Shale
• Maotianshan shales
• Ediacaran biota

[edit] Further reading

• Budd, G. E. & Jensen, J. (2000). "A critical reappraisal of the fossil record of the bilaterian phyla.". Biological Reviews 75: 253–295. doi:10.1017/S000632310000548X. 
• Collins, Allen G. “Metazoa: Fossil record”. Retrieved Dec. 14, 2005.
• Conway Morris, S. (1997). The Crucible of Creation: the Burgess Shale and the rise of animals. Oxford University Press. ISBN 0-19-286202-2
• Conway Morris, S. (2006). "Darwin’s dilemma: the realities of the Cambrian ‘explosion’". Philosophical Transactions of the Royal Society B: Biological Sciences 361 (1470): 1069–1083. doi:10.1098/rstb.2006.1846.  An enjoyable account.
• Kennedy, M., M. Droser, L. Mayer., D. Pevear, and D. Mrofka (2006). "Clay and Atmospheric Oxygen". Science 311 (5766): 1341. doi:10.1126/science.311.5766.1341c. 
• Knoll,A. H. and Carroll, S. B. (1999). Early Animal Evolution: Emerging Views from Comparative Biology and Geology. Science 284 (5423): 2129 – 2137.
• Alexander V. Markov, and Andrey V. Korotayev (2007) “Phanerozoic marine biodiversity follows a hyperbolic trend” Palaeoworld 16(4): pp. 311-318.
• Parker, A. (2004). In the Blink of an Eye, Free Press, ISBN 0-7432-5733-2.
• Wang, D. Y.-C., S. Kumar and S. B. Hedges (1999). "Divergence time estimates for the early history of animal phyla and the origin of plants, animals and fungi". Proceedings of the Royal Society of London, Series B, Biological Sciences 266 (1415): 163–71. doi:10.1098/rspb.1999.0617. 
• Xiao, S., Y. Zhang, and A. Knoll (1998). "Three-dimensional preservation of algae and animal embryos in a Neoproterozoic phosphorite". Nature 391: 553–58. doi:10.1038/35318. 

Timeline References:

• Gradstein and Ogg, “A Phanerozoic time scale”, v.19, no.1&2., 1996.
• Martin, M.W.; Grazhdankin, D.V.; Bowring, S.A.; Evans, D.A.D.; Fedonkin, M.A.; Kirschvink, J.L. (2000). "Age of Neoproterozoic Bilaterian Body and Trace Fossils, White Sea, Russia: Implications for Metazoan Evolution". Science 288: 841–845. doi:10.1126/science.288.5467.841. 

[edit] External links

• The Cambrian “explosion” of metazoans and molecular biology: would Darwin be satisfied?
• On embryos and ancestors by Stephen Jay Gould
• The Cambrian “explosion”: Slow-fuse or megatonnage?
• The Cambrian Explosion – In Our Time, BBC Radio 4 broadcast, 17 February 2005
• Utah's Cambrian life - new (2008) website with good images of a range of Burgess-shale-type and other Cambrian fossils.

[edit] References